Tin electroplating electrolyte



United States Patent US. Cl. 20454 1 Claim ABSTRACT OF THE DISCLOSURE An electrolyte for electroplating tin containing as brightener an additive composition made by sulfonating a mixture of phenols and lower alkyl phenols wherein the lower alkyl phenols comprise l-% of the phenols present.

This invention relates to highly water-soluble additive compositions composed mostly of dihydroxydiphenylsultone and effective as additives for acid electrolytic tinplating baths and to their electroplating baths.

More particularly the present invention relates to additive compositions for acid electrolytic tin-plating baths wherein the water-solubility is improved without impairing the inherent performance as a lustering agent of dihydroxydiphenylsulfone which is hard to dissolve in water, by adding a small amount of a reforming agent in producing dihydroxydiphenylsulfonate.

Dihydroxydiphenylsulfone (which shall be abbreviated merely as DDS hereinafter) is at present a substance industrially useful as an additive for electrolytic tin-plating baths but is regretfully poor in water solubility. For example, the water-solubility of its known product is less than 3.0 g./l. at C. This property caused an industrially diflicult problem, the solution of which has ever been longed for, in case it was to be used as an additive for acid baths for electrolytic tin-plating iron and steel.

That is to say, in the case of electrolytic tin-plating a steel plate, for the purpose of making a lustrous tinplated surface while making the plated surface smooth, DDS has been added to an acid electrolytic tin-plating bath composed mostly of phenolsulfonic acid and tin sulfate and an electrolytic tin-plating has been carried out in such plating bath. However, as the solubility of DDS in the electroplating bath is so low, its crystals are often deposited during plating and specifically the adjustment of the concentration of the electroplating bath becomes difiicult.

Moreover, when tin-plating a steel sheet with an acid tin-plating electrolyte, rise of the bath temperature is an inevitable incident, which makes it necessary to cool the electroplating bath in circulation to keep it always at a proper temperature. However, when DDS dissolved in the electroplating bath at an elevated temperature is cooled, it has defects that it is easily deposited in the electroplating bath, causing clog of an electroplating bath circulating pipe for cooling which results in a reduction of the cooling efliciency of the electroplating bath.

A usually conceived means of improving the Watersolubility of DDS is an addition of a surface active agent. However, a simultaneous application of an entirely heterogenous substance is not desired. Even in case it has no bad influence due to the heterogeneity, a higher cost of production will be inevitable.

As a result of studying method of increasing the Watersolubility of DDS without impairing its inherent property in view of the above mentioned points, I have reached the present invention.

The present invention is to improve the water-solubility of DDS by replacing 1 to 20 wt. percent of phenol used 3,483,099 Patented Dec. 9, 1969 ice in producing DD-S with a lower alkyl phenol which is its derivative of homologous series.

The phenol and lower alkyl phenol are so close to each other in the structure and chemical characteristics and the amounts of alkylphenol in use is very slight so such additive composition for electroplating baths as is obtained by the present invention does not impair the inherent performance of DDS.

Further, one of the features of the present invention is that excepting the partial replacement of the raw material phenol to be used with a lower alkyl phenol, the other reaction conditions are exactly identical with the producing conditions of DDS.

Consequently, as compared with conventional DDS, there are no particular troubles in the producing operation according to the line method of the present invention and it has become possible to easily produce highly water-soluble additive compositions for electroplating baths at substantially the same cost.

The conventional method of producing DDS is as follows:

to parts of 98% sulfuric acid or fuming sulfuric acid are dropped and mixed below 100 C. into 188 parts of molten phenol. The mixture is then quickly heated to C. and thereafter gradually up to to 200 C. during the time of from 3 to 6 hours, until the reaction is concluded.

In the course of the reaction, water and a small amount of phenol come to be distilled out, of which the water should be removed out of the reaction system, while the phenol should be returned into the reaction system. The reaction product thus obtained is refined with hot water of above 95 C. in an amount about 10 times as large as that of the reaction product.

In the method of producing the additive composition for electroplating baths of the present invention, an amount corresponding to 1 to 20 wt. percent of 188 parts of phe- 1101 to be used in the reaction is only replaced with a lower alkyl phenol, but the other conditions are the same as in the above.

The low alkyl phenols here are cresol, ethyl phenol, propyl phenol (including those of isomers of alkyl radicals) and butyl phenol (including those of isomers of alkyl radicals).

As the most desirable example of the lower alkyl phenol to be used in the present invention, the case of cresol shall be explained.

As well known, thereare three kinds of cresol: ortho, meta and para isomers. Any one of them or a mixture of two or three of them is effective to obtain a composition high in water-solubility. Their use is included in the present invention. But, orthocresol is the most effective among them, and paracresol and metacresol are less effective in the order mentioned. Now, the water-solubilities of the additive compositions for electroplating baths of the present invention in case respective kinds of cresol were added to phenol are shown in Table 1.

TABLE 1 (at 25 0.)

Rate of replacement of phenol According to the results shown in Table 1 it is ascertained that the higher the rate of replacement of phenol with cresol, the better the improvement of the watersolubility. However, on the other hand, when the amount The cresol to be added to the phenol must be more than i 1 wt. percent. As shown in Table 1, the solubility rises remarkably until the cresol is 5%, but the degree of the improvement in the solubility is relatively low when 10% is exceeded.

For the above mentioned reasons, the proper rate of replacement of phenol with cresol in the present invention will be 1 to 20%, but preferably 1 to 10% from the industrial point of view.

The cresol to be used as a raw material may be any of para, ortho and meta isomers or a mixture of such isomers. However, it is preferable to use para or orthocresol, because metacresol is not only the least effective among them, but also diflicult to be got industrially.

The additive composition for electroplating baths of the present invention is thought to contain two or more of alkyl dihydroxydiphenylsulfones represented by R 56 HO OH and/ or dialkyl dihydroxydiphenylsulfones represented by R R H O II -Q HO OH The substance of the additive composition for electroplating baths of the present invention will be as mentioned above. However, no remarkable improvement of the water solubility of the additive composition can be obtained by simply mixing DDS and dihydroxydimethyldiphenylsultone and adding the thus obtained mixture into an acid electrolytic tin-plating bath. An eifect of largely improving the water-solubility of the additive composition may be obtained only when in DDS a part of phenol is replaced with cresol and the mixture is sulfonated. The reason for this can not be elucidated. However, it is a wonderful discovery that, when a part of the raw material phenol was only replaced with cresol, an unexpected improvement of the solubility could be seen.

Table 2 shows the water-solubilities of mixtures at various ratios of DDS and dihydroxydimethyldiphenylsulfone (synthesized by using an ortho compound) at 25 C.

TABLE 2 (at 25 0.)

Rate of mixing Dihydroxydiphenylsultone 100 90 95 90 80 (DDS), percent. Rate of mixing Dihydroxydimethyldipheu- 0 1 5 ylsulfone, percent.

Water-solubility of mixtures of DDS and 3.0 3.0 3.2 3.5 3.8

Dlilhydroxydimethyldiphenylsulione, g.

rate of replacement of the raw material phenol with a lower alkyl phenol is in the range of from 10 to 15%. However, from the viewpoint of the industrial production it will be in the range of from 1 to 10% as already mentioned.

When the additive composition for electroplating baths 3f the present invention is added to a plating bath, it becomes possible to maintain a bath temperature favorable by cooling the plating bath in circulation, the plated surface is not made rough by deposited crystals, whereby a beautiful tin-plated plate may be obtained. The amount of the additive composition to be added to an acid electrolytic tin-plating bath, which is composed of phenolsulfonic acid and tin sulfate, is not specifically limited. However, when adding 10 to 15 g./'l. of the additive composition of the present invention to the acid electrolytic tin-plating bath, more effect may be displayed in lustering the tinplated surface.

The electroplating conditions are sufficient with the ordinary plating conditions.

As the electroplating bath temperature will rise to 50 to 60 C. during the plating, the plating bath must be constantly circulated to a cooler to maintain the plating bath temperature at 40 C. However, in the case of using the acid tinplating bath prepared by adding the additive composition of the present invention, even when the bath is excessively cooled in the cooler, the additive composition of the present invention, which is a surface smoothing substance, will be scarcely deposited.

Now, examples of the preparation of additive compositions for electroplating baths of the present invention and examples of the application thereof to an electrolytic tinplating shall be enumerated in the following.

Example 1 of the preparation of the additive composition for plating baths of the present invention:

parts (by weight) of 25% fuming sulfuric acid were dropped and mixed below C. into a mixture of 178 parts (by weight) of phenol and 10 parts (by weight) of orthocresol. The mixture was then quickly heated to C. Then, while the water coming distilled out was being removed out of the reaction system and the phenol coming distilled out was being returned into the reaction system, the temperature of the mixure was gradually elevated during 3 to 6 hours. When the temperature finally reached 195 C., the reaction was completed and then the reaction product was refined.

The Water-solubility shows a remarkable improvement as compared with the water-solubility of 3.0 g./l. at 25 C. of a DDS product prepared from only phenol as a raw material.

Example 2 of the preparation of the additive composition for plating baths of the present invention:

100 parts (by weight) of 98 sulfuric acid were dropped and mixed below 100 C. into a mixture of 186 parts (by weight) of phenol and 2 parts (by weight) of paracresol. The mixture was then quickly heated to 165 C. Then, while the water coming distilled out was being removed out of the reaction system and the phenol coming distilled out was being returned into the reaction system, the emperature of the mixture was gradually elevaed 3 to 6 hours. When the temperature finally reached 200 C., the reaction was completed and then the reaction product was refined.

The water-solubility of the additive composition for plating baths of the present invention obtained by this reaction was 4.3 g./l. at 25 C.

This water-solubility shows an improvement as compared with the water-solubility of 3.0 g./l. of a DDS product prepared from only phenol as a raw material.

Example 3 of the preparation of the additive composition for plating baths of the present invention:

93 parts (by weight) of 25 fuming sulfuric acid were dropped and mixed below 100 C. into a mixture of parts (by weight) of phenol and 18 parts (by weight) of industrial crude cresol (which was a mixture of a lower alkyl phenol composed mostly of metacresol, paracresol and orthocresol). The mixture was then quickly heated to 165 C. Then, while the water coming distilled out was being removed out of the reaction system and the phenol coming distilled out was being returned into the reaction system, the temperature of the mixture was gradually elevated during 3 to 6 hours. When the temperature finally reached 190 C., the reaction was completed and then the reaction product was refined.

The water-solubility of the additive composition for plating baths of the present invention obtained by his reaction was 7.0 g./l. at 25 C.

This water-solubility shows a remarkable improvement as compared with the water-solubility of 3.0 g./l, at 25 C. of a DDS product prepared from only phenol as a raw material.

Example 1 of the application:

A cold-rolled steel strip, the surface of which has been cleaned, was electroplated with an acid tin-plating solution of the following bath composition:

Electroplating plating bath composition:

Phenolsulfonic acid (g./l.) Tin sulfate (g./l.) 30 Additive composition for plating baths of the present invention obtained by sulfonating phenol as mixed with 5% orthocresol (g./l.) Current densities (a./dm. 10, and Temperature (C.)

The plated thicknesses of the tin-plated steel plates obtained under the above mentioned conditions were 0.4, 0.8 and 1.6 respectively. The plated surfaces were milkwhite, smooth and fine, just like those obtained when using the conventional smoothing agent.

The plating bath was cooled in circulation so that its temperature might be 40 C. The additive was deposited so little not only in the plating bath tub but also in the cooler that the amount of deposition was only /5 as large as in the case of using a conventional additive.

Example 2 of the application:

In the test tin-platings carried out with additive compositions of the present invention under the same conditions as in Example 1 the kinds and amounts of cresol mixed in the raw material phenol were varied to clarify 4 industrial cresol used in this test is a commercial mixture of ortho, meta and para cresols.

TABLE 3 Degree of Deposition Ratio of Crystal Deposition 1 Kinds of Cresol 1 Ratio of crystal deposition (R.C.D.) is calculated as follows:

Crystal deposition amount in the plating bath system when using the additive composition containing any kind of cresol R.C.D.

Crystal deposition amount in the plating bath system when using DDS.

Thus, a sufficient effect can be expected with the amount of addition of cresol of less than 15% to phenol.

What is claimed is:

1. In an aqueous acidic electroplating bath for electroplating bright tin comprising tin ions, the improvement in said bath comprising having present an additive composition obtained by mixing to parts of 98% sulfuric acid with 188 parts of a mixture of a phenol and a lower alkyl phenol wherein said lower alkyl phenol comprises 120% by weight of said mixture and sulfonating the resulting mixture by quickly heating the same up to C. and thereafter gradually up to C. to 200 C. in the time of 3-6 hrs. until the reaction is concluded.

References Cited UNITED STATES PATENTS 2,179,038 11/1939 Guthke 26049 XR 2,313,371 3/1943 Stack 20454 2,399,194 4/1946 Andrews 204-54 2,633,450 3/1953 Andrews 20454 FOREIGN PATENTS 1,353,309 1/1964 France.

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R. 26049 

